1. Field of the Invention
The present invention relates to a device for detecting the angular position of the rotor of a rotary electric machine such as an electric motor with electronic switching.
2. Description of the Prior Art
Generally a rotary electric machine is formed from a stator and a rotor equipped with two respective mutually facing magnetic circuits each comprising a plurality of magnetic poles of alternating polarity.
Thus, in some of these machines, the stator is designed so as to induce a stationary magnetic field, whereas the rotor is provided with armature windings creating a rotating magnetic field. The interaction between the magnetic field of the rotor and that of the stator, staggered with respect to each other, forms an electromagnetic couple. In order to form a couple with a constant direction, the direction of the current in each armature winding must change when it passes from one magnetic pole to another of the stator, of opposite polarity. This change is generally provided by means of a switch adapted for supplying the armature coils with power depending on the relative position of the magnetic fields of the inductor and of the armature and of a given angular shift.
Conversely, in another type of machine, the stator is provided with inductor windings adapted for generating a rotating magnetic field, whereas the rotor is provided with poles having a constant magnetic field. In this case, the machine comprises a switch for changing the direction of the current in the inductor windings depending on the angular position of the rotor.
In one case as in the other, the switch must necessarily involve a device for supplying with the power the different windings whose field is to be modified, a device for detecting the angular position of the poles of the rotor and a selector which determines the supply mode for these windings depending on said angular position and on the desired direction of rotation.
For detecting the angular position of the poles of the rotor, numerous solutions have been proposed, these solutions involving more especially magnetic sensors, capacitive sensors, optical sensors or even Hall effect sensors.
For this latter type of sensor, a solution generally used consists in mounting in the stator a Hall effect cell which supplies a voltage representative of the position of the rotor by means of its own rotating field. However, this solution is not always applicable and does not allow good accuracy to be obtained.
Another solution consists in using a coded disk, made from a ferromagnetic material mounted at the end of the rotor shaft and, disposed facing this disk and on each side thereof, a permanent magnet and a Hall effect cell. One drawback of this circuit is that, under the effect of the magnetic field of the permanent magnet, the ferromagnetic material disk is the seat of eddy currents which risk disturbing the operation of the motor. Furthermore, because of the presence of the disk, the gap between the permanent magnet and the cell is relatively large, which leads to using relatively powerful magnets which risk generating magnetic parasites, and high sensitivity cells.